Generally, in the field of dust collection by means of bag filters, the filter cloth must be possessed not only of the capability of catching dust, but also high mechanical strength and form retention capability because the cloth must be subjected to a periodical mechanical shaking operation to remove the dust which has accumulated in the cloth so as to reactivate the same.
Thus, most conventional filter cloths in the bag filters are either a woven cloth or felt.
On the other hand, in the field of dust collection by means of bag filters these days, the dust is becoming more and more finer, as exemplified by dye, cement or resin powder, etc. It is becoming more and more difficult to efficiently cope with such fine powder-like dust by means of conventional filter cloths. Namely, as shown in FIGS. 6(a) and 6(b), in a conventional filter cloth 4a, the pores 2a, constituted by the fibers 1a forming a filtering surface 7a, are relatively large, and the fiber fusions 3a, caused by singeing or pressing, occupy relatively large two-dimensional areas. Therefore, conventional filtering mechanisms, which rely upon dust accumulation within the cloth, are prone to clogging and the shaking off of the dust is difficult, which results in higher pressure drop and shorter useful life.
Consequently, the problems associated with the conventional filter cloth are: (a) the pores in the filtering surface are too large to catch fine particles on the filtering surface; and (b) separability (clearing) of dust is inadequate.
In an attempt to improve the above-mentioned dust separability, it has been proposed to treat the filter cloth with a fluorocarbon or silicone resin. The so-treated filter cloth is in fact effective in its early stage of use; however, its durability is rather poor. Thus, it does not provide a satisfactory solution.
By way of another possible solution, it has been proposed, with a view of forming minute pores on the surface, to provide a coating to the filter cloth, the coating being that of a urethane resin or foamed acrylic resin. However, they have a rather low porosity and high pressure drop and result in insufficient performance. In addition, such processing requires higher manufacturing costs.
Furthermore, there is a filter cloth laminated with a membrane on its surface, the membrane being obtained by spreading a PTFE (tetrafluoroethylene resin) film, typically known as "GOATEX" (trade name of Goatex Co.). The thus obtained filter cloth, because of the superior dust separability due to the minute pores and the function of the resin, may be called the best filter cloth at the present time. However, it has a drawback in that its manufacturing cost is very high. Moreover, the anti-wear property of the membrane is rather inferior and it is prone to breakage. Therefore, if recovery of a powder product is intended, it poses a serious problem in that the fragments of the membrane that fall off are mixed with the product.
On the other hand, in use as air filters for automobiles or for air conditioners, a filter cloth comprising a unitary layer of nonwoven fabric formed by the so-called spun bond method or melt blow method, or comprising composite layers of nonwoven fabric formed by the spun bond method and the melt blow method respectively, is widely used. Out of these new types of filter cloth, the nonwoven fabric produced by the melt blow method is formed of much finer fibers as compared with conventional ones, and the pores formed as a result of the entanglement of the fibers are small enough to seize fine particles. However, it is rather difficult to utilize the above-mentioned filter cloth made of nonwoven fabric as a filter cloth for use in bag filters. The reason is that such cloth is not possessed of the strength and form retention capability necessary for withstanding the high dust load as well as the repeated mechanical reactivation operation common in bag filters.
In order to cope with this problem, it is conceivable to laminate a nonwoven fabric produced by the melt blow method to the surface of felt by means of needle punching. However, by so doing, the minute pores formed by the melt blow method are likely to be enlarged by the needle punching, rendering the attempt rather meaningless. Moreover, since the nonwoven fabric formed by the melt blow method is made of ultrafine fibers, it may not be adequately bonded with the felt layer which functions as a base material.
Even if one tries to bond them together using fine, rapidly moving fluids, such as water jet punching in place of needle punching, it is not possible to bond them strongly, and moreover, the fluids tend to leave traces on the surface of the layer made of the ultrafine fibers, making the surface rugged which will adversely affect the separability of dust.